THIS invention relates to an air transfer valve. The invention is particularly concerned with an air transfer valve, otherwise known as an air release valve, which can be used to vent air from a pipeline in which a liquid is conveyed under pressure, or from other pressurised vessels.
A known air transfer or release valve has a housing with an inlet at its lower end which is connected, typically at a high point, to a pipeline from which air is to be vented. The housing has an outlet at its upper end and in its simplest form accommodates a float. During pipeline filling, the theory is that water or other liquid in the pipeline will enter the housing and buoy up the float to seal against the outlet. Air which then accumulates in the housing during operation of the pipeline will cause the float to lose buoyancy with the result that it drops away from the outlet to allow accumulated air to vent through the outlet to atmosphere.
In practice however, it has been observed that in many cases, the float may be “blown” closed during rapid pipeline filling and then be held in a closed position by the pressure in the pipeline. This may for instance occur in a situation where the valve is installed at a high point in the pipeline which is lower than another high point further downstream. During initial filling of the pipeline, air accumulating at the high point served by the valve will vent through the valve according to theory, but if such venting is not fast enough, an air pocket may be formed beneath the valve. The air in this pocket will be pressurised rapidly by the pumping pressure and the downstream head of liquid.
The pressure differential acting across the valve float may in some instances be sufficient to “blow” the float to a closed position dynamically. The internal pipeline pressure may then maintain the float in this closed position, preventing further venting through the valve.
Similarly, if the float has been hydraulically closed, i.e. it has been buoyed up by liquid in the housing to seal against the outlet, it may remain in this condition by internal pressure in the pipeline. Still further it is possible, during rapid pipeline filling, for liquid to be lifted up by a bubble of air which enters the housing from the pipeline, and this can also cause the float to seal against the outlet. In any of these situations, the float may then be held closed by the elevated internal pressure forces until the pipeline is fully depressurised.
To address the problem of venting in internally pressurised situations it has been proposed to provide a small bleed orifice through the float to allow equalisation of the internal pressure. However a bleed orifice can only vent air very slowly and so is unable to achieve pressure equalisation fast enough for the float to drop down to allow the required rapid venting of air to take place.
A known air transfer or release valve is described in the specification of South African patent 87/5645 (ARI Flow Control Accessories). This valve has a narrow outlet slot which is covered by a flexible strip attached to a float. Downward movement of the float in response to air accumulation unpeels the strip incrementally from the slot and allows small volumes of trapped air to vent from the valve housing. A problem with this design is that only small volumes can be vented through the narrow slot.
Other solutions which have been proposed make use of mechanical linkages. One example is described in the specification of U.S. Pat. No. 5,090,439 (APCO). The air release valve described in this specification operates such that when the valve is closed, pressure-related forces-acting on the closure are balanced.
Only a buoyancy force acting on a float is operative to hold the valve closure in a closed position. This facilitates valve opening even under pressurised conditions. However the design is such that when the float loses buoyancy and the valve closure does open, the pressure in the float housing, which may be quite considerable, generates an unbalanced force on the closure tending to hold it open.
It is an objective of the present invention to provide an alternative air release valve which addresses the problem of air venting in a pressurised system.